Alkali-free borosilicate glass compositions have been used as the glass compositions for the substrate of information display devices, particularly active-matrix liquid crystal display devices. A representative example of such alkali-free borosilicate glass is Code 7059 glass produced by Corning of the United States.
In manufacturing processes of glass compositions, a process that removes residual bubbles in the glass composition is generally known as fining. A method of adding a fining agent to fine a glass melt is commonly known as well. Arsenic oxide, antimony oxide, and fluorides are some of the known examples of the fining agent. In this connection, there is a social need to reduce the amount of these highly environmentally unfriendly components. For example, JP 10(1998)-25132A discloses a method in which sulfate and chloride are simultaneously added to raw glass materials for the purposes of melting and fining.
Specifically, this publication teaches adding, as fining agents, sulfate in the SO3 amount of 0.005 to 1.0 weight %, and chloride in the Cl2 amount of 0.01 to 2.0 weight %. The publication also teaches preparing sulfate and chloride as fining agents, and using BaSO4, CaSO4, and the like as the sulfate, and BaCl2, CaCl2, and the like as the chloride.
Meanwhile, JP 60(1985)-141642A discloses a glass composition containing as a degassing agent at least one component selected from As2O3, Sb2O3, (NH4)2SO4, NaCl, and fluorides.
In the glass fining technique disclosed in JP 10(1998)-25132A, sulfate and chloride simultaneously are added to raw glass materials. The chloride added is an alkali earth metal chloride such as BaCl2, CaCl2.
However, the alkali-free borosilicate glass composition commonly used for the active-matrix liquid crystal display device is highly viscous and this has made fining of the glass difficult.
Further, components such as aluminum, boron, and silicon have characteristics that their mobility in glass is limited by the strong electrostatic binding due to the large charge. For this reason, it has been difficult to achieve good clarity in the glass of the borosilicate glass composition.
As a countermeasure to these problems, JP 60(1985)-141642A teaches using NaCl. A problem of using NaCl, however, is that sodium ions dissolve out of the glass after the glass is assembled into a liquid crystal display device, and, depending on the amount of dissolved sodium ions, the performance of the liquid crystal element may be impaired.